Biochip spots single viruses

By
Eric Smalley,
Technology Research NewsEnvironmental
sensors and handheld devices that quickly and easily detect and identify
individual viruses would provide early warning of infections in individuals,
the spread of disease in populations, and biological weapons attacks.

The rapid development of nanotechnology in recent years has given
researchers tools for building highly sensitive virus detectors. A team
from Harvard University has built a detector from nanowires transistors
that can identify individual virus particles in real time in unpurified
samples.

The researchers' prototype uses antibody proteins attached to the
nanowires to briefly capture individual virus particles. "The binding causes
a change in the current of the nanowire-based electronic device, which signals
the virus presence," said Charles Lieber, a professor of chemistry at Harvard
University.

Labs-on-a-chip that are based on the device could be used to monitor
diseases, said Lieber. For example, the AIDS virus, like other viruses,
begins to duplicate itself after it enters a cell, he said. "If [these additional
pathogens] can be spotted and treated by drugs before they overwhelm the
body's immune system, there is less likelihood that the infection will turn
into full-blown AIDS."

The device could also be used to study how viruses bind to receptors
by determining which viruses bind to which receptors, how long virus particles
bind to receptors, and what substances block or disrupt binding, Lieber
said. The device could also eventually be used to detect individual biomolecules,
including DNA and proteins, he said.

The researchers made their prototype by growing 20-nanometer-diameter
silicon nanowires, mixing the nanowires with fluid, and flowing them into
position across nickel contacts spaced two microns apart to form nanowire
transistors. They coated the nanowires with aldehyde, then added antibody
proteins, which adhered to the aldehyde. They configured a microfluidic
channel to flow fluid containing the viruses across the nanowire sensors.

When an individual virus binds to a nanowire transistor antibody
receptor, the transistor's electrical conductance changes, increasing or
decreasing depending on whether the transistor carries positive or negative
charge and the virus is positively or negatively charged. "Our detection
method is based on a pure electrical detection of selective binding-unbinding
of a single viral particle," said Lieber.

The researchers found that the duration of the bind-and-release
cycle depends on the density of the antibody proteins on the nanowires.
The cycle averaged just over one second at a low concentration of antibody
proteins, about 20 seconds at a moderate density, and 5 to 10 minutes at
a high density.

The researchers attached antibody proteins that bind to influenza
A to one portion of the nanowire array and antibody proteins that bind to
adenovirus to another portion and demonstrated that they can detect simultaneous
binding of the two different types of viruses. "These nanowire detectors...
could be scaled easily to enable sensing thousands of different viruses
simultaneously," said Lieber.

Researchers have been able to detect individual virus particles
in the laboratory for years using optical microscopes, but these approaches
require purifying the samples and labeling the viruses with fluorescent
markers, which makes them inappropriate for clinical and field diagnostics,
said Lieber. They are also not able to detect viruses in samples with very
low virus concentrations or detect multiple types of viruses, he said.

In recent years, researchers have also developed methods of detecting
viruses using nanowire-and nanotube-based transistors, but these approaches
involve processing purified samples to break down virus particles so that
the virus DNA can bind to receptors on the devices.

Another technique that has emerged recently is detecting individual
virus particles using microscopic cantilevers coated with antibody receptors.
The additional mass of an attached virus particle changes the vibration
rate of the cantilever, which can be detected electrically. These techniques
require high-resolution imaging to confirm that only one virus particle
has attached to the cantilever, however, said Lieber.

The researchers' next step is scaling up the nanowire transistor
device, said Lieber. "We are working on a larger detector array, one that
could sense up to 100 different viruses simultaneously," he said.

The researchers' sensor arrays could be used practically in two
to five years, said Lieber.

Lieber's research colleagues were Fernando Patolsky, Gengfeng Zheng,
Oliver Hayden, Melike Lakadamyali, and Xiaowei Zhuang. The work appeared
in the September 13, 2004 issue of the Proceedings of the National Association
of Science. The research was funded by the Defense Advanced Research
Projects Agency (DARPA), the National Cancer Institute, the Ellison Medical
Foundation, the Office of Naval Research, and the Searle Scholar Program.